Meldonium was put on the World Anti-Doping Agency's list of banned substances in January, and Sharapova was the highest-profile athlete to test positive for it. The five-time Grand Slam champ said that she took the drug over a ten-year period to treat irregular heart test results, early indications of diabetes and multiple bouts with the flu — which are the label uses for the drug.

"Depending on the patient's health condition, treatment course of meldonium preparations may vary from four to six weeks. Treatment course can be repeated twice or thrice a year," the company, Grindeks, said in a statement to The Associated Press. "Only physicians can follow and evaluate patient's health condition and state whether the patient should use meldonium for a longer period of time."

Meldonium was banned in January and there has been a string of failed tests by athletes in several sports since, but none with a bigger profile than Maria Sharapova. (James Ellingworth/AP)

Anti-doping experts say that using meldonium can increase oxygen intake, leading to improved stamina and endurance.

Sharapova, who called the result "a huge mistake" on her part, could face a ban of one-year from the International Tennis Federation, though former WADA chief Dick Pound speculated that she could see a four-year suspension, which could signal the end the 28-year-old's career.